Array substrate for display device and method of fabricating the same

ABSTRACT

An array substrate for a display device includes: a substrate; first and second gate electrodes of impurity-doped polycrystalline silicon on the substrate; a gate insulating layer on the first and second gate electrodes; first and second active layers of intrinsic polycrystalline silicon on the gate insulating layer, the first and second active layers corresponding to the first and second active layers, respectively; an interlayer insulating layer on the first and second active layers and including first to fourth active contact holes, the first and second active contact holes exposing side portions of the first active layer, the third and fourth active contact holes exposing side portions of the second active layer; first and second ohmic contact layers of impurity-doped amorphous silicon on the interlayer insulating layer, the first ohmic contact layer contacting the first active layer through the first and second active contact holes, the second ohmic contact layer contacting the second active layer through the third and fourth active contact hole; first source and drain electrodes on the first ohmic contact layer and second source and drain electrodes on the second ohmic contact layer; a data line on the interlayer insulating layer, the data line connected to the first source electrode; a first passivation layer on the first source and drain electrodes, the second source and drain electrodes and the data line; a gate line on the first passivation layer, the gate line connected to the first gate electrode and crossing the data line to define a pixel region; a second passivation layer on the gate line; and a pixel electrode on the second passivation layer, the pixel electrode connected to the second drain electrode.

The present invention claims the benefit of Korean Patent Application No. 10-2009-0094007, filed in Korea on Oct. 1, 2009, which is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an array substrate for a display device, and more particularly, to an array substrate including a thin film transistor and a storage capacitor and a method of fabricating the array substrate.

2. Discussion of the Related Art

As information technology progresses, flat panel display (FPD) devices having light weight, thin profile and low power consumption have been developed. Specifically, a cathode ray tube (CRT) has been replaced by the FPD devices such as a liquid crystal display (LCD) device or an organic electroluminescent display (ELD) device.

The LCD devices use the optical anisotropy and polarization properties of liquid crystal molecules to produce an image. Among various types LCD devices, an active matrix type liquid crystal display (AM-LCD) device where a thin film transistor (TFT) is formed in each pixel region as a switching element turning on/off a voltage has been the subject of recent research due to its high resolution and superior quality for displaying moving images.

The organic ELD devices have a high brightness and a low driving voltage. In addition, since the organic ELD devices are an emissive type, the organic ELD devices have a high contrast ratio and a thin profile. Also, the organic ELD devices can display images without viewing angle limitations. Further, since the organic ELD devices have a short response time of several microseconds (μs), the organic ELD devices have a stable operation property at a low temperature and an advantage in displaying moving images. Moreover, since the organic ELD devices are driven with a relatively low voltage of about 5 V to about 15 V, the organic ELD devices have advantages in design and fabrication of a driving circuit.

Each of an AM-LCD device and an organic ELD device includes an array substrate having a thin film transistor (TFT) in each pixel region as a switching element for controlling a data signal application. Specifically, the array substrate of the organic ELD device further has a driving TFT for driving an organic electroluminescent (EL) diode in each pixel region.

FIG. 1 is a cross-sectional view showing an array substrate for a display device according to the related art. In FIG. 1, a gate electrode 15 and a gate line (not shown) are formed on a substrate 11 having a pixel region P, and a gate insulating layer 18 is formed on the gate electrode 15 and the gate line. A semiconductor layer 28 including an active layer 22 of intrinsic amorphous silicon and an ohmic contact layer 26 of impurity-doped amorphous silicon is formed on the gate insulating layer 18 over the gate electrode 15. In addition, source and drain electrodes 36 and 38 spaced apart from each other are formed on the ohmic contact layer 26, and a data line 33 is formed over the gate insulating layer 18. A semiconductor pattern 29 including first and second patterns 27 and 23 is formed between the data line 33 and the gate insulating layer 18.

The gate electrode 15, the gate insulating layer 18, the semiconductor layer 28, the source electrode 36 and the drain electrode 38 constitute a thin film transistor (TFT) Tr. When the display device is an organic electroluminescent display device, the TFT Tr may function as a driving TFT, and a switching TFT (not shown) connected to the gate line, the data line 33 and the TFT Tr may be formed in the pixel region P. Further, a passivation layer 42 is formed on the source and drain electrodes 36 and 38, and a pixel electrode 50 is formed on the passivation layer 42 in the pixel region P. The passivation layer 42 includes a drain contact hole 45 exposing the drain electrode 38, and the pixel electrode 50 is connected to the drain electrode 38 through the drain contact hole 45.

In the TFT Tr, the active layer 22 of the semiconductor layer 28 includes a first portion exposed between the ohmic contact layer 26 and a second portion under the ohmic contact layer 26. The first portion of the active layer 22 has a first thickness t1 and the second portion of the active layer 22 has a second thickness t2 different from the first thickness t2 (t1≠t2). The difference between the first and second thicknesses t1 and t2 cause deterioration of the TFT Tr.

FIG. 2 is a cross-sectional showing a step of forming a semiconductor layer, a source electrode and a drain electrode of an array substrate for a display device according to the related art. Although not shown in FIG. 2, an active layer, an impurity-doped amorphous silicon pattern and a source-drain pattern are formed on a substrate 11 by patterning an intrinsic amorphous silicon layer, an impurity-doped amorphous silicon layer and a metal layer. Next, source and drain electrodes 36 and 38 are formed by patterning the source-drain pattern, and the impurity-doped amorphous silicon pattern is exposed between the source and drain electrodes 36 and 38.

In FIG. 2, the impurity-doped amorphous silicon pattern exposed between the source and drain electrodes 36 and 38 is removed by a dry etching method to form an ohmic contact layer 26 under the source and drain electrodes 36 and 38. When the dry etching step is performed for a insufficient time, the impurity-doped amorphous silicon pattern exposed between the source and drain electrodes 36 and 38 may remain on the active layer 26 and the remaining impurity-doped amorphous silicon pattern may deteriorate a thin film transistor (TFT) Tr (of FIG. 1). Accordingly, the dry etching step is performed for a sufficient time to remove the impurity-doped amorphous silicon pattern exposed between the source and drain electrodes 36 and 38 completely and the active layer 22 under the impurity-doped amorphous silicon pattern is partially removed. As a result, the active layer 22 includes a first portion that is exposed between the source and drain electrodes 36 and 38 and has a first thickness t1 and a second portion that is under the ohmic contact layer 26 and has a second thickness t2 different from the first thickness (t1≠t2).

However, the thickness difference between the first and second portions of the active layer 22 deteriorates the TFT Tr. In addition, since the intrinsic amorphous silicon layer is formed to have a sufficient thickness over about 1000 Å based on the partial removal of the active layer 22 in the dry etching step, the deposition time for the intrinsic amorphous silicon layer increases and productivity is reduced.

In an array substrate for a display device such as an AM-LCD device and an organic ELD device, a thin film transistor (TFT) is connected to a gate line, a data line and a pixel electrode in each pixel region and applies a data signal to the pixel electrode selectively and periodically. The TFT includes an active layer and the active layer may be formed of amorphous silicon or polycrystalline silicon. Since amorphous silicon has a random atomic arrangement, amorphous silicon has a transition from a stable state to a quasi-stable state when light is irradiated or when an electric field is applied. Accordingly, the TFT including an active layer of amorphous silicon has problems in stability. In addition, since amorphous silicon has relatively low carrier mobility within a range of about 0.1 cm²/Vs to about 1.0 cm²/Vs in a channel region, the TFT including an active layer of amorphous silicon has problems in use as an element of a driving circuit.

To solve the above problems of amorphous silicon, the TFT including an active layer of polycrystalline silicon has been suggested. For example, amorphous silicon for a semiconductor layer may be crystallized to become polycrystalline by a crystallization method using a laser apparatus.

FIG. 3 is a cross-sectional view showing an array substrate including a polycrystalline silicon thin film transistor according to the related art. In FIG. 3, a buffer layer 53 is formed on a substrate 51, and a semiconductor layer 55 of polycrystalline silicon is formed on the buffer layer 53. The semiconductor layer 55 includes an active region 55 a of intrinsic polycrystalline silicon and a source-drain region 55 b of impurity-doped polycrystalline silicon at both sides of the active region 55 a. The source-drain region 55 b may include one of negative type impurities of high concentration (n+) and positive type impurities of high concentration (p+).

A gate insulating layer 58 is formed on the semiconductor layer 55, and a gate electrode 59 is formed on the gate insulating layer 58 over the semiconductor layer 55. In addition, an interlayer insulating layer 61 is formed on the gate electrode 59, and source and drain electrodes 70 and 72 are formed on the interlayer insulating layer 61. The interlayer insulating layer 61 and the gate insulating layer 58 include first and second semiconductor contact holes 63 and 64 exposing the source-drain region 55 b, and the source and drain electrodes 70 and 72 are connected to the source-drain region 55 b through the first and second semiconductor contact holes 63 and 64, respectively. A passivation layer 75 is formed on the source and drain electrodes 70 and 72, and a pixel electrode 82 is formed on the passivation layer 75. The passivation layer 75 includes a drain contact hole 78 exposing the drain electrode 72, and the pixel electrode 82 is connected to the drain electrode 78 through the drain contact hole 78.

The source-drain region 55 b may be formed by doping the semiconductor layer of intrinsic polycrystalline silicon with the impurities, and the doping step may be performed using an ion implantation apparatus. Accordingly, an additional apparatus is required for fabricating a polycrystalline silicon TFT and fabrication cost increases.

Further, when a bottom gate structure where a gate electrode is under a semiconductor layer is applied to a polycrystalline silicon TFT, a crystallization step having a relatively high process temperature is performed for a semiconductor layer of amorphous silicon over a gate electrode. Since the gate electrode is formed of a metallic material, the gate electrode may be degraded during the crystallization step.

Moreover, as a display device having higher quality and higher resolution is required, a size of a pixel region is reduced. As a result, a sufficient capacitance of a storage capacitor without reduction of aperture ratio has been the subject of recent research and development.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to an array substrate for a display device and a method of fabricating the same that substantially obviate one or more of the problems due to limitations and disadvantages of the related art.

An object of the present invention is to provide an array substrate for a display device where a property of a thin film transistor is improved by protecting an active layer during a dry etching step and forming a semiconductor layer of polycrystalline silicon without a doping step and a method of fabricating the array substrate.

Another object of the present invention is to provide an array substrate for a display device where a thin film transistor has a bottom gate structure due to a gate electrode of polycrystalline silicon and a method of fabricating the array substrate.

Another object of the present invention is to provide an array substrate for a display device where a sufficient capacitance of a storage capacitor is obtained without reduction of aperture ratio and a method of fabricating the array substrate.

Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, an array substrate for a display device includes: a substrate; first and second gate electrodes of impurity-doped polycrystalline silicon on the substrate; a gate insulating layer on the first and second gate electrodes; first and second active layers of intrinsic polycrystalline silicon on the gate insulating layer, the first and second active layers corresponding to the first and second active layers, respectively; an interlayer insulating layer on the first and second active layers and including first to fourth active contact holes, the first and second active contact holes exposing side portions of the first active layer, the third and fourth active contact holes exposing side portions of the second active layer; first and second ohmic contact layers of impurity-doped amorphous silicon on the interlayer insulating layer, the first ohmic contact layer contacting the first active layer through the first and second active contact holes, the second ohmic contact layer contacting the second active layer through the third and fourth active contact hole; first source and drain electrodes on the first ohmic contact layer and second source and drain electrodes on the second ohmic contact layer; a data line on the interlayer insulating layer, the data line connected to the first source electrode; a first passivation layer on the first source and drain electrodes, the second source and drain electrodes and the data line; a gate line on the first passivation layer, the gate line connected to the first gate electrode and crossing the data line to define a pixel region; a second passivation layer on the gate line; and a pixel electrode on the second passivation layer, the pixel electrode connected to the second drain electrode.

In another aspect, a method of fabricating an array substrate for a display device includes: sequentially forming a first impurity-doped amorphous silicon layer, a first inorganic insulating layer and an intrinsic amorphous silicon layer on a substrate; crystallizing the first impurity-doped amorphous silicon layer and the intrinsic amorphous silicon layer to form an impurity-doped polycrystalline silicon layer and an intrinsic polycrystalline silicon layer; patterning the impurity-doped polycrystalline silicon layer, the first organic insulating layer and the intrinsic polycrystalline silicon layer to form a first gate electrode, a gate insulating layer and a first active layer of polycrystalline silicon sequentially on the substrate and a second gate electrode, the gate insulating layer and a second active layer of polycrystalline silicon sequentially on the substrate; forming an interlayer insulating layer on the first and second active layers, the interlayer insulating layer including first to fourth active contact holes, the first and second active contact holes exposing side portions of the first active layer, the third and fourth active contact holes exposing side portions of the second active layer; sequentially forming a second impurity-doped amorphous silicon layer and a first metal layer on the interlayer insulating layer; pattering the second impurity-doped amorphous silicon layer and the first metal layer to form first and second ohmic contact layers, first source and drain electrodes on the first ohmic contact layer, second source and drain electrodes on the second ohmic contact layer and a data line on the interlayer insulating layer, the first ohmic contact layer contacting the first active layer through the first and second active contact holes, the second ohmic contact layer contacting the second active layer through the third and fourth active contact hole; forming a first passivation layer on the first source and drain electrodes, the second source and drain electrodes and the data line; forming a gate line on the first passivation layer, the gate line connected to the first gate electrode and crossing the data line to define a pixel region; forming a second passivation layer on the gate line; and forming a pixel electrode on the second passivation layer, the pixel electrode connected to the second drain electrode.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. In the drawings:

FIG. 1 is a cross-sectional view showing an array substrate for a display device according to the related art;

FIG. 2 is a cross-sectional showing a step of forming a semiconductor layer, a source electrode and a drain electrode of an array substrate for a display device according to the related art;

FIG. 3 is a cross-sectional view showing an array substrate including a polycrystalline silicon thin film transistor according to the related art;

FIG. 4 is a plan view showing an array substrate for an organic electroluminescent display device according to a first embodiment of the present invention;

FIG. 5 is a cross-sectional view, which is taken along a line V-V of FIG. 4, showing an array substrate for an organic electroluminescent display device according to a first embodiment of the present invention;

FIG. 6 is a cross-sectional view showing an array substrate for an organic electroluminescent display device according to a second embodiment of the present invention;

FIG. 7 is a cross-sectional view showing an array substrate for an organic electroluminescent display device according to a third embodiment of the present invention;

FIG. 8 is a cross-sectional view showing an array substrate for an organic electroluminescent display device according to a fourth embodiment of the present invention;

FIG. 9 is a cross-sectional view showing an array substrate for an organic electroluminescent display device according to a fifth embodiment of the present invention; and

FIG. 10 is a cross-sectional view showing an array substrate for an organic electroluminescent display device according to a sixth embodiment of the present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Reference will now be made in detail to the illustrated embodiments of the present invention, which are illustrated in the accompanying drawings.

FIG. 4 is a plan view showing an array substrate for an organic electroluminescent display device according to a first embodiment of the present invention.

In FIG. 4, a gate line 145, a data line 130 and a power line 148 are formed on a substrate 101. The gate line 145 and the data line 130 cross each other to define a pixel region P, and the power line 148 is spaced apart from the gate line 145. A power electrode 134 connected to the power line 148 through a power contact hole 154 is formed to be parallel to the data line 130.

In addition, a switching thin film transistor (TFT) STr and a driving TFT DTr connected to each other are formed in the pixel region P. The switching TFT STr includes a first gate electrode 105 a, a first active layer 115 a, a first source electrode 133 a and a first drain electrode 136 a, and the driving TFT DTr includes a second gate electrode 105 b, a second active layer 115 b, a second source electrode 133 b and a second drain electrode 136 b. The first gate electrode 105 a is connected to the gate line 145 through a first gate contact hole 142 a, and the first source electrode 133 a is connected to the data line 130. A gate auxiliary pattern 146 is connected to the second gate electrode 105 b through a second gate contact hole 142 b, and a gate connecting electrode 172 is connected to the first drain electrode 136 a through a first drain contact hole 152 a and to the gate auxiliary pattern 146 through an auxiliary contact hole 153. The second source electrode 133 b extends to the power electrode 134.

Further, a pixel electrode 170 is formed in the pixel region, and an organic luminescent layer (not shown) is formed on the pixel electrode 170. The pixel electrode 170 is connected to the second drain electrode 136 b through a second drain contact hole 152 b. The organic luminescent layer emits one of red, green and blue colored lights and the organic luminescent layers emitting different colored lights may be alternately disposed in the pixel regions P. A reference electrode (not shown) is formed on the organic luminescent layer. The reference electrode may be formed over an entire surface of the substrate 101.

The power electrode 134 overlaps the second gate electrode 105 b with a gate insulating layer (not shown) and an interlayer insulating layer (not shown) interposed therebetween to constitute a storage capacitor StgC. In addition, the pixel electrode 170, the organic luminescent layer and the reference electrode constitute an organic electroluminescent (EL) diode (not shown). For example, the pixel electrode 170 may function as an anode of the organic EL diode and include a transparent conductive material having a relatively high work function. The reference electrode may function as a cathode of the organic EL diode and include a metallic material having a relatively low work function. Although not shown in FIG. 4, a bank of one of inorganic and organic insulating materials is formed at a boundary portion of the pixel region P. The bank overlaps the pixel electrode 170 to separate the organic luminescent layers in adjacent pixel regions P and to prevent an electric shortage between the pixel electrode 170 and the reference electrode.

FIG. 5 is a cross-sectional view, which is taken along a line V-V of FIG. 4, showing an array substrate for an organic electroluminescent display device according to a first embodiment of the present invention.

In FIG. 5, a buffer layer 102 of an inorganic insulating material is formed on an entire surface of a substrate 101, and first and second gate electrodes 105 a and 105 b each including an impurity-doped polycrystalline silicon are formed on the buffer layer 102. A gate insulating layer 109 of an inorganic insulating material is formed on the first and second gate electrodes 105 a and 105 b, and first and second active layers 115 a and 115 b each including an intrinsic polycrystalline silicon are formed on the gate insulating layer 109. The first and second active layers 115 a and 115 b are formed over the first and second gate electrodes 105 a and 105 b, respectively.

In addition, an interlayer insulating layer 122 including first to fourth active contact holes 123 a, 123 b, 123 c and 123 d is formed on the first and second active layers 115 a and 115 b. The first and second active contact holes 123 a and 123 b expos side portions of the first active layer 115 a, respectively, and the third and fourth active contact holes 123 c and 123 d expose side portions of the second active layer 115 b, respectively. First and second ohmic contact layers 127 a and 127 b of impurity-doped amorphous silicon are formed on the interlayer insulating layer 122. The first ohmic contact layer 127 a is connected to the side portions of the first active layer 115 a through the first and second active contact holes 123 a and 123 b, and the second ohmic contact layer 127 b is connected to the side portions of the second active layer 115 b through the third and fourth active contact holes 123 c and 123 d. A first source electrode 133 a and a first drain electrode 136 a spaced apart from each other are formed on the first ohmic contact layer 127 a, and a second source electrode 133 b and a second drain electrode 136 b spaced apart from each other are formed on the second ohmic contact layer 127 b. In addition, a power electrode 134 is formed on the second ohmic contact layer 127 b. The second source electrode 133 b extends to the power electrode 134 and the power electrode 134 is connected to a power line 148 (of FIG. 4). A data line 130 (of FIG. 4) connected to the first source electrode 133 a is formed on the interlayer insulating layer 122.

The power electrode 134 and the second gate electrode 105 b overlap each other with the gate insulating layer 109 and the interlayer insulating layer 122 interposed therebetween to constitute a storage capacitor StgC. In addition, the first gate electrode 105 a, the gate insulating layer 109, the first active layer 115 a, the first ohmic contact layer 127 a, the first source electrode 133 a and the first drain electrode 136 a constitute a switching thin film transistor (TFT) STr, and the second gate electrode 105 b, the gate insulating layer 109, the second active layer 115 b, the second ohmic contact layer 127 b, the second source electrode 133 b and the second drain electrode 136 b constitute a driving TFT DTr.

Further, a first passivation layer 140 of an inorganic insulating material is formed on the first source electrode 133 a, the first drain electrode 136 a, the second source electrode 133 b, the second drain electrode 136 b and the data line. The first passivation layer 140, the interlayer insulating layer 122 and the gate insulating layer 109 include first and second gate contact holes 142 a and 142 b exposing the first and second gate electrodes 105 a and 105 b, respectively. Although not shown in FIG. 5, the first passivation layer 140 includes a power contact hole exposing the power electrode 134 extending from the second source electrode 133 b.

A gate line 145 and a gate auxiliary pattern 146 are formed on the first passivation layer 140. The gate line 145 crosses the data line to define a pixel region P and is connected to the first gate electrode 105 a through the first gate contact hole 142 a. The gate auxiliary pattern 146 is connected to the second gate electrode 105 b through the second gate contact hole 142 b. The power line 148 (of FIG. 4) parallel to the gate line 145 is formed on the first passivation layer 140. The power line 148 is connected to the power electrode 134 through the power contact hole.

Moreover, a second passivation layer 150 of an inorganic insulating material is formed on the gate line 145 and the gate auxiliary pattern 146. The second passivation layer 150 includes an auxiliary contact hole 153 exposing the gate auxiliary pattern 146. In addition, the second passivation layer 150 and the first passivation layer 140 include first and second drain contact holes 152 a and 152 b exposing the first and second drain electrodes 136 a and 136 b, respectively.

A pixel electrode 170 and a gate connecting electrode 172 each including a transparent conductive material are formed on the second passivation layer 150. The pixel electrode 170 is connected to the second drain electrode 136 b through the second drain contact hole 152 b. In addition, the gate connecting electrode 172 is connected to the first drain electrode 136 a through the first drain contact hole 152 a and to the gate auxiliary pattern 146 through the auxiliary contact hole 153.

Although not shown in FIG. 5, a bank of one of inorganic and organic insulating materials is formed at a boundary portion of the pixel region P. The bank may be formed on the second passivation layer 150 and the gate connecting electrode 172. In addition, an organic luminescent layer is formed on the pixel electrode 170 and a reference electrode is formed on the organic luminescent layer. The organic luminescent layer emits one of red, green and blue colored lights and the organic luminescent layers emitting different colored lights may be alternately disposed in the pixel regions P. The reference electrode may be formed over an entire surface of the substrate 101. The pixel electrode 170, the organic luminescent layer and the reference electrode in the pixel region P constitute an organic electroluminescent (EL) diode.

A method of fabricating an array substrate for an organic electroluminescent display device according to a first embodiment of the present invention will be illustrated hereinafter with reference to FIGS. 4 and 5.

In FIGS. 4 and 5, the buffer layer 102 is formed on the substrate 101 by depositing an inorganic insulating material such as silicon nitride (SiNx) and silicon oxide (SiO₂). Impurities such as alkali ions may erupt from the substrate 101 during a subsequent crystallization step for the first and second active layers 615 a and 615 b, for example, a solid phase crystallization (SPC) step with a process temperature of about 600° C. to about 800° C., and the first and second active layers 115 a and 115 b of polycrystalline silicon may be deteriorated by the impurities. The buffer layer 102 shields the impurities and prevents the deterioration of the first and second active layers 115 a and 115 b.

A first impurity-doped amorphous silicon layer (not shown), a first inorganic insulating layer (not shown) and an intrinsic amorphous silicon layer (not shown) are sequentially formed on the buffer layer 102 by sequentially depositing impurity-doped amorphous silicon, an inorganic insulating material such as silicon nitride (SiNx) and silicon oxide (SiO₂) and intrinsic amorphous silicon. Since channel regions of the first and second active layers 115 a and 115 b are protected by the interlayer insulating layer 122 during the dry etching step for the first to fourth active contact holes 123 a, 123 b, 123 c and 123 d, the thickness difference of the first and second active layers 115 a and 115 b is prevented. Accordingly, the intrinsic amorphous silicon layer may be formed to have a thickness of about 300 Å to about 1000 Å.

Next, an impurity-doped polycrystalline silicon layer (not shown) and an intrinsic polycrystalline silicon layer (not shown) are formed by crystallizing the first impurity-doped amorphous silicon layer and the intrinsic amorphous silicon layer through the solid phase crystallization (SPC) method. For example, a thermal crystallization method under a process temperature of about 600° C. to about 800° C. or an alternating magnetic field crystallization method under a process temperature of about 600° C. to 700° C. may be used for the crystallization.

Next, the first gate electrode 105 a, the gate insulating layer 109 and a first polycrystalline silicon pattern (not shown) and the second gate electrode 105 b, the gate insulating layer 109 and a second polycrystalline silicon pattern (not shown) are formed by patterning the impurity-doped polycrystalline silicon layer, the first inorganic insulating layer and the intrinsic polycrystalline silicon layer. The first gate electrode 105 a, the gate insulating layer 109 and the first polycrystalline silicon pattern have the same shape as each other, and the second gate electrode 105 b, the gate insulating layer 109 and the second polycrystalline silicon pattern have the same shape as each other.

In a bottom gate type polycrystalline silicon TFT, a gate electrode of a metallic material is deteriorated during the crystallization step for an active layer. For example, the gate electrode of a metallic material may be deformed or the metallic material may be diffused through the gate insulating layer into the active layer, which is referred to as a spiking, during the crystallization step having a process temperature of about 600° C. Since the first and second gate electrodes 105 a and 105 b are formed of impurity-doped amorphous silicon, deterioration of the first and second gate electrodes 105 a and 105 b during the crystallization step is prevented.

Next, the first and second active layers 115 a and 115 b of intrinsic polycrystalline silicon are formed by patterning the first and second intrinsic polycrystalline silicon patterns. In addition, the interlayer insulating layer 122 is formed on the first and second active layers 115 a and 115 b by depositing an inorganic insulating material such as silicon nitride (SiNx) and silicon oxide (SiO₂), and the first, second, third and fourth active contact holes 123 a, 123 b, 123 c and 123 d are formed by patterning the interlayer insulating layer 122. The first and second active contact holes 123 a and 123 b expose the side portions of the first active layer 115 a, and the third and fourth active contact holes 123 c and 123 d expose the side portions of the second active layer 115 b. Since the interlayer insulating layer 122 covering the central portion of the first and second active layers 115 a and 115 b functions as an etch stopper during the step of forming the first, second, third and fourth active contact holes 123 a, 123 b, 123 c and 123 d, the thickness difference or the deterioration of the first and second active layers 115 a and 115 b is prevented.

Next, a second impurity-doped amorphous silicon layer (not shown) is formed on the interlayer insulating layer 122 by depositing impurity-doped amorphous silicon, and a first metal layer (not shown) is formed on the second impurity-doped amorphous silicon layer by depositing a first metallic material such as molybdenum (Mo), chromium (Cr) and molybdenum titanium (MoTi). The data line (not shown), the first ohmic contact layer 127 a, the first source electrode 133 a, the first drain electrode 136 a, the second ohmic contact layer 127 b, the second source electrode 133 b, the second drain electrode 136 b, the power electrode 134 are formed by patterning the first metal layer and the second impurity-doped amorphous silicon layer. Each of the data line and the power electrode 134 includes a dummy pattern of impurity-doped amorphous silicon. The first ohmic contact layer 127 a contacts the first active layer 115 a through the first and second active contact holes 123 a and 123 b, and the second ohmic contact layer 127 b contacts the second active layer 115 b through the third and fourth active contact holes 123 c and 123 d. The first source and drain electrodes 133 a and 136 a are formed on the first ohmic contact layer 127 a, and the second source and drain electrodes 133 b and 136 b are formed on the second ohmic contact layer 127 b.

In another embodiment, a barrier layer of intrinsic amorphous silicon may be formed under the second impurity-doped amorphous silicon layer. The barrier layer may be disposed between the first active layer 115 a and the first ohmic contact layer 127 a in the first and second active contact holes 123 a and 123 b and between the second active layer 115 b and the second ohmic contact layer 127 b in the third and fourth active contact holes 123 c and 123 d. Since the adhesion between intrinsic polycrystalline silicon and intrinsic amorphous silicon is better than the adhesion between intrinsic polycrystalline silicon and impurity-doped amorphous silicon, the adhesion and the contact resistance between each of the first and second active layers 115 a and 115 b and each of the first and second ohmic contact layers 127 a and 127 b may be improved by the barrier layer. The barrier layer may be patterned together with the first and second ohmic contact layers 127 a and 127 b to have the same shape as the ohmic contact layers 127 a and 127 b.

Moreover, before the first and second ohmic contact layers 127 a and 127 b or the barrier layer is formed on the interlayer insulating layer 122, a cleaning step using a buffered oxide etchant (BOE) may be performed for the substrate 101 having the interlayer insulating layer. During the crystallization step having a process temperature of about 600° C. to about 800° C., a surface of the first and second active layers 115 a and 115 b is oxidized and a thin thermal oxide film is formed on the first and second active layers 115 a and 115 b. The contact property between each of the first and second active layers 115 a and 115 b and each of the first and second ohmic contact layers 127 a and 127 b or between each of the first and second active layers 115 a and 115 b and the barrier layer may be degraded by the thin thermal oxide film. Accordingly, the thin thermal oxide film exposed through the first, second, third and fourth active contact holes 123 a, 123 b, 123 c and 123 d may be removed through the cleaning step using a BOE.

The first gate electrode 105 a, the gate insulating layer 109, the first active layer 115 a, the first ohmic contact layer 127 a, the first source electrode 133 a and the first drain electrode 136 a constitute a switching thin film transistor (TFT) STr, and the second gate electrode 105 b, the gate insulating layer 109, the second active layer 115 b, the second ohmic contact layer 127 b, the second source electrode 133 b and the second drain electrode 136 b constitute a driving TFT DTr. In addition, the second gate electrode 105 b, the gate insulating layer 109, the interlayer insulating layer 122 and the power electrode 134 overlapping each other constitute a storage capacitor StgC.

Next, the first passivation layer 140 is formed on the first source electrode 133 a, the first drain electrode 136 a, the second source electrode 133 b, the second drain electrode 136 b and the data line by depositing an inorganic insulating material such as silicon nitride (SiNx) and silicon oxide (SiO₂), and first and second gate contact holes 142 a and 142 b are formed by patterning the first passivation layer 140, the interlayer insulating layer 122 and the gate insulating layer 109. The first and second gate contact holes 142 a and 142 b expose the first and second gate electrodes 105 a and 105 b, respectively. In addition, the power contact hole 154 exposing the power electrode 134 is formed by patterning the first passivation layer 140.

Next, a second metal layer (not shown) is formed on the first passivation layer by depositing a second metallic material such as aluminum (Al), aluminum alloy (e.g., aluminum neodymium (AlNd)), copper (Cu), copper alloy, molybdenum (Mo) and chromium (Cr), and the gate line 145, the gate auxiliary pattern 146 and the power line 148 are formed by patterning the second metal layer. The power line 148 is connected to the power electrode 134 through the power contact hole 154.

Next, the second passivation layer 150 is formed on the gate line 145, the gate auxiliary pattern 146 and the power line 148 by depositing an inorganic insulating material such as silicon nitride (SiNx) and silicon oxide (SiO₂), and the first and second drain contact holes 152 a and 152 b are formed by patterning the second passivation layer 150 and the first passivation layer 140. The first and second drain contact holes 152 a and 152 b expose the first and second drain electrodes 136 a and 136 b, respectively. In addition, the auxiliary contact hole 153 exposing the gate auxiliary pattern 146 is formed by patterning the second passivation layer 150.

Next, the pixel electrode 170 and the gate connecting electrode 172 are formed on the second passivation layer 150 by depositing and patterning a transparent conductive material such as indium-tin-oxide (ITO) and indium-zinc-oxide (IZO). The pixel electrode 170 is connected to the second drain electrode 136 b through the second drain contact hole 152 b, and the gate connecting electrode 172 is connected to the first drain electrode 136 a through the first drain contact hole 152 a and to the gate auxiliary pattern 146 through the auxiliary contact hole 153.

Next, the bank (not shown) is formed on an edge portion of the pixel electrode 170 by depositing or coating one of inorganic and organic insulating materials, and the organic luminescent layer (not shown) is formed on the bank and the pixel electrode 170. In addition, the reference electrode (not shown) is formed on the organic luminescent layer.

In the organic ELD device according to the first embodiment of the present invention, since the first and second active layers 115 a and 115 b include intrinsic polycrystalline silicon, a mobility of the switching TFT STr and the driving TFT DTr is improved. In addition, since the first and second gate electrodes 105 a and 105 b include impurity-doped polycrystalline silicon, the crystallization step for the first and second active layers 115 a and 115 b is performed without deformation of the first and second gate electrodes 105 a and 105 b and a polycrystalline silicon TFT having a bottom gate structure is obtained. Further, the interlayer insulating layer 122 on a central portion of the first and second active layers 115 a and 115 b functions as an etch stopper for a channel region. Accordingly, the first and second active layers 115 a and 115 b have a uniform thickness and deterioration of the switching TFT STr and the driving TFT DTr due to the thickness difference is prevented. Moreover, since the first and second ohmic contact layers 127 a and 127 b are formed of impurity-doped amorphous silicon, a doping step using an ion implantation apparatus is omitted and initial investment for an additional apparatus is reduced.

Further, the power electrode 134 extending from the second source electrode 133 b and connected to the power line 148 (of FIG. 4) overlaps the second gate electrode 105 b with the gate insulating layer 109 and the interlayer insulating layer 122 interposed therebetween to constitute the storage capacitor StgC. As the resolution of the organic ELD device increases, a size of the storage capacitor StgC decreases and a capacitance of the storage capacitor StgC also decreases. In another embodiment, a structure of the storage capacitor StgC is changed to obtain a sufficient capacitance.

FIG. 6 is a cross-sectional view showing an array substrate for an organic electroluminescent display device according to a second embodiment of the present invention.

In FIG. 6, a buffer layer 202 of an inorganic insulating material is formed on an entire surface of a substrate 201, and first and second gate electrodes 205 a and 205 b each including an impurity-doped polycrystalline silicon are formed on the buffer layer 202. A gate insulating layer 209 of an inorganic insulating material is formed on the first and second gate electrodes 205 a and 205 b, and first and second active layers 215 a and 215 b each including an intrinsic polycrystalline silicon are formed on the gate insulating layer 209. The first and second active layers 215 a and 215 b are formed over the first and second gate electrodes 205 a and 205 b, respectively.

In addition, an interlayer insulating layer 222 including first to fourth active contact holes 223 a, 223 b, 223 c and 223 d is formed on the first and second active layers 215 a and 215 b. The first and second active contact holes 223 a and 223 b expos side portions of the first active layer 215 a, respectively, and the third and fourth active contact holes 223 c and 223 d expose side portions of the second active layer 215 b, respectively. First and second ohmic contact layers 227 a and 227 b of impurity-doped amorphous silicon are formed on the interlayer insulating layer 222. The first ohmic contact layer 227 a is connected to the side portions of the first active layer 215 a through the first and second active contact holes 223 a and 223 b, and the second ohmic contact layer 227 b is connected to the side portions of the second active layer 215 b through the third and fourth active contact holes 223 c and 223 d. A first source electrode 233 a and a first drain electrode 236 a spaced apart from each other are formed on the first ohmic contact layer 227 a, and a second source electrode 233 b and a second drain electrode 236 b spaced apart from each other are formed on the second ohmic contact layer 227 b. In addition, a power electrode 234 is formed on the second ohmic contact layer 227 b. The second source electrode 233 b extends to the power electrode 234 and the power electrode 234 is connected to a power line (not shown). Although not shown in FIG. 6, a data line connected to the first source electrode 233 a is formed on the interlayer insulating layer 222.

The power electrode 234 and the second gate electrode 205 b overlap each other with the gate insulating layer 209 and the interlayer insulating layer 222 interposed therebetween to constitute a first storage capacitor StgC1. In addition, the first gate electrode 205 a, the gate insulating layer 209, the first active layer 215 a, the first ohmic contact layer 227 a, the first source electrode 233 a and the first drain electrode 236 a constitute a switching thin film transistor (TFT) STr, and the second gate electrode 205 b, the gate insulating layer 209, the second active layer 215 b, the second ohmic contact layer 227 b, the second source electrode 233 b and the second drain electrode 236 b constitute a driving TFT DTr.

Further, a first passivation layer 240 of an inorganic insulating material is formed on the first source electrode 233 a, the first drain electrode 236 a, the second source electrode 233 b, the second drain electrode 236 b and the data line. The first passivation layer 240, the interlayer insulating layer 222 and the gate insulating layer 209 include first and second gate contact holes 242 a and 242 b exposing the first and second gate electrodes 205 a and 205 b, respectively. Although not shown in FIG. 6, the first passivation layer 240 includes a power contact hole exposing the power electrode 234 extending from the second source electrode 233 b.

A gate line 245 and a gate auxiliary pattern 246 are formed on the first passivation layer 240. The gate line 245 crosses the data line to define a pixel region P and is connected to the first gate electrode 205 a though the first gate contact hole 242 a. The gate auxiliary pattern 246 is connected to the second gate electrode 205 b through the second gate contact hole 242 b. In addition, the gate auxiliary pattern 246 overlaps the power electrode 234 with the first passivation layer 240 interposed therebetween to constitute a second storage capacitor StgC2. Although not shown in FIG. 6, the power line parallel to the gate line 245 is formed on the first passivation layer 240 and is connected to the power electrode 234 through the power contact hole.

Moreover, a second passivation layer 250 of an inorganic insulating material is formed on the gate line 245 and the gate auxiliary pattern 246. The second passivation layer 250 includes an auxiliary contact hole 253 exposing the gate auxiliary pattern 246. In addition, the second passivation layer 250 and the first passivation layer 240 include first and second drain contact holes 252 a and 252 b exposing the first and second drain electrodes 236 a and 236 b, respectively.

A pixel electrode 270 and a gate connecting electrode 272 each including a transparent conductive material are formed on the second passivation layer 250. The pixel electrode 270 is connected to the second drain electrode 236 b through the second drain contact hole 252 b. In addition, the gate connecting electrode 272 is connected to the first drain electrode 236 a through the first drain contact hole 252 a and to the gate auxiliary pattern 246 through the auxiliary contact hole 253.

Although not shown in FIG. 6, a bank of one of inorganic and organic insulating materials is formed at a boundary portion of the pixel region P. The bank may be formed on the second passivation layer 250 and the gate connecting electrode 272. In addition, an organic luminescent layer is formed on the pixel electrode 270 and a reference electrode is formed on the organic luminescent layer. The organic luminescent layer emits one of red, green and blue colored lights and the organic luminescent layers emitting different colored lights may be alternately disposed in the pixel regions P. The reference electrode may be formed over an entire surface of the substrate 201. The pixel electrode 270, the organic luminescent layer and the reference electrode in the pixel region P constitute an organic electroluminescent (EL) diode.

In the organic ELD device according to the second embodiment of the present invention, since the first and second active layers 215 a and 215 b include intrinsic polycrystalline silicon, a mobility of the switching TFT STr and the driving TFT DTr is improved. In addition, since the first and second gate electrodes 205 a and 205 b include impurity-doped polycrystalline silicon, the crystallization step for the first and second active layers 215 a and 215 b is performed without deformation of the first and second gate electrodes 205 a and 205 b and a polycrystalline silicon TFT having a bottom gate structure is obtained. Further, the interlayer insulating layer 222 on a central portion of the first and second active layers 215 a and 215 b functions as an etch stopper for a channel region. Accordingly, the first and second active layers 215 a and 215 b have a uniform thickness and deterioration of the switching TFT STr and the driving TFT DTr due to the thickness difference is prevented. Moreover, since the first and second ohmic contact layers 227 a and 227 b are formed of impurity-doped amorphous silicon, a doping step using an ion implantation apparatus is omitted and initial investment for an additional apparatus is reduced.

Further, the power electrode 234 extending form the second source electrode 233 b overlaps the second gate electrode 205 b with the gate insulating layer 209 and the interlayer insulating layer 222 interposed therebetween to constitute the first storage capacitor StgC1, and the gate auxiliary pattern 246 connected to the second gate electrode 205 b overlaps the power electrode 234 extending from the second source electrode 233 b with the first passivation layer 240 interposed therebetween to constitute the second storage capacitor StgC2. Since the first and second storage capacitors StgC1 and StgC2 are connected to the second source electrode 233 b of the driving TFT DTr in parallel, the total storage capacitance of the organic ELD device increases.

FIG. 7 is a cross-sectional view showing an array substrate for an organic electroluminescent display device according to a third embodiment of the present invention.

In FIG. 7, a buffer layer 302 of an inorganic insulating material is formed on an entire surface of a substrate 301, and first and second gate electrodes 305 a and 305 b each including an impurity-doped polycrystalline silicon are formed on the buffer layer 302. A gate insulating layer 309 of an inorganic insulating material is formed on the first and second gate electrodes 305 a and 305 b, and first and second active layers 215 a and 315 b each including an intrinsic polycrystalline silicon are formed on the gate insulating layer 309. The first and second active layers 315 a and 315 b are formed over the first and second gate electrodes 305 a and 305 b, respectively.

In addition, an interlayer insulating layer 322 including first to fourth active contact holes 323 a, 323 b, 323 c and 323 d is formed on the first and second active layers 315 a and 315 b. The first and second active contact holes 323 a and 323 b expos side portions of the first active layer 315 a, respectively, and the third and fourth active contact holes 323 c and 323 d expose side portions of the second active layer 315 b, respectively. First and second ohmic contact layers 327 a and 327 b of impurity-doped amorphous silicon are formed on the interlayer insulating layer 322. The first ohmic contact layer 327 a is connected to the side portions of the first active layer 315 a through the first and second active contact holes 323 a and 323 b, and the second ohmic contact layer 327 b is connected to the side portions of the second active layer 315 b through the third and fourth active contact holes 323 c and 323 d. A first source electrode 333 a and a first drain electrode 336 a spaced apart from each other are formed on the first ohmic contact layer 327 a, and a second source electrode 333 b and a second drain electrode 336 b spaced apart from each other are formed on the second ohmic contact layer 327 b. In addition, a power electrode 334 is formed on the second ohmic contact layer 327 b. The second source electrode 333 b extends to the power electrode 334 and the power electrode 334 is connected to a power line (not shown). Although not shown in FIG. 7, a data line connected to the first source electrode 333 a is formed on the interlayer insulating layer 322.

The power electrode 334 and the second gate electrode 305 b overlap each other with the gate insulating layer 309 and the interlayer insulating layer 322 interposed therebetween to constitute a first storage capacitor StgC1. In addition, the first gate electrode 305 a, the gate insulating layer 309, the first active layer 315 a, the first ohmic contact layer 327 a, the first source electrode 333 a and the first drain electrode 336 a constitute a switching thin film transistor (TFT) STr, and the second gate electrode 305 b, the gate insulating layer 309, the second active layer 315 b, the second ohmic contact layer 327 b, the second source electrode 333 b and the second drain electrode 336 b constitute a driving TFT DTr.

Further, a first passivation layer 340 of an inorganic insulating material is formed on the first source electrode 333 a, the first drain electrode 336 a, the second source electrode 333 b, the second drain electrode 336 b and the data line. The first passivation layer 340, the interlayer insulating layer 322 and the gate insulating layer 309 include first and second gate contact holes 342 a and 342 b exposing the first and second gate electrodes 305 a and 305 b, respectively. Although not shown in FIG. 7, the first passivation layer 340 includes a power contact hole exposing the power electrode 334 extending from the second source electrode 333 b.

A gate line 345 and a gate auxiliary pattern 346 are formed on the first passivation layer 340. The gate line 345 crosses the data line to define a pixel region P and is connected to the first gate electrode 305 a through the first gate contact hole 342 a. The gate auxiliary pattern 346 is connected to the second gate electrode 305 b through the second gate contact hole 342 b. Although not shown in FIG. 7, the power line parallel to the gate line 345 is formed on the first passivation layer 340 and is connected to the power electrode 334 through the power contact hole.

Moreover, a second passivation layer 350 of an inorganic insulating material is formed on the gate line 345 and the gate auxiliary pattern 346. The second passivation layer 350 includes an auxiliary contact hole 353 exposing the gate auxiliary pattern 346. In addition, the second passivation layer 350 and the first passivation layer 340 include first and second drain contact holes 352 a and 352 b exposing the first and second drain electrodes 336 a and 336 b, respectively.

A pixel electrode 370 and a gate connecting electrode 372 each including a transparent conductive material are formed on the second passivation layer 350. The pixel electrode 370 is connected to the second drain electrode 336 b through the second drain contact hole 352 b. The gate connecting electrode 372 is connected to the first drain electrode 336 a through the first drain contact hole 352 a and to the gate auxiliary pattern 346 through the auxiliary contact hole 353. In addition, the gate connecting electrode 372 overlaps the power electrode 334 with the second passivation layer 350 and the first passivation layer 340 interposed therebetween to constitute a second storage capacitor StgC2.

Although not shown in FIG. 7, a bank of one of inorganic and organic insulating materials is formed at a boundary portion of the pixel region P. The bank may be formed on the second passivation layer 350 and the gate connecting electrode 372. In addition, an organic luminescent layer is formed on the pixel electrode 370 and a reference electrode is formed on the organic luminescent layer. The organic luminescent layer emits one of red, green and blue colored lights and the organic luminescent layers emitting different colored lights may be alternately disposed in the pixel regions P. The reference electrode may be formed over an entire surface of the substrate 301. The pixel electrode 370, the organic luminescent layer and the reference electrode in the pixel region P constitute an organic electroluminescent (EL) diode.

In the organic ELD device according to the third embodiment of the present invention, since the first and second active layers 315 a and 315 b include intrinsic polycrystalline silicon, a mobility of the switching TFT STr and the driving TFT DTr is improved. In addition, since the first and second gate electrodes 305 a and 305 b include impurity-doped polycrystalline silicon, the crystallization step for the first and second active layers 315 a and 315 b is performed without deformation of the first and second gate electrodes 305 a and 305 b and a polycrystalline silicon TFT having a bottom gate structure is obtained. Further, the interlayer insulating layer 322 on a central portion of the first and second active layers 315 a and 315 b functions as an etch stopper for a channel region. Accordingly, the first and second active layers 315 a and 315 b have a uniform thickness and deterioration of the switching TFT STr and the driving TFT DTr due to the thickness difference is prevented. Moreover, since the first and second ohmic contact layers 327 a and 327 b are formed of impurity-doped amorphous silicon, a doping step using an ion implantation apparatus is omitted and initial investment for an additional apparatus is reduced.

Further, the power electrode 334 extending form the second source electrode 333 b overlaps the second gate electrode 305 b with the gate insulating layer 309 and the interlayer insulating layer 322 interposed therebetween to constitute the first storage capacitor StgC1, and the gate connecting electrode 372 connected to the second gate electrode 305 b through the gate auxiliary pattern 346 overlaps the power electrode 334 extending from the second source electrode 333 b with the second passivation layer 350 and the first passivation layer 340 interposed therebetween to constitute the second storage capacitor StgC2. Since the first and second storage capacitors StgC1 and StgC2 are connected to the second source electrode 333 b of the driving TFT DTr in parallel, the total storage capacitance of the organic ELD device increases.

FIG. 8 is a cross-sectional view showing an array substrate for an organic electroluminescent display device according to a fourth embodiment of the present invention.

In FIG. 8, a buffer layer 402 of an inorganic insulating material is formed on an entire surface of a substrate 401, and first and second gate electrodes 405 a and 405 b each including an impurity-doped polycrystalline silicon are formed on the buffer layer 402. A gate insulating layer 409 of an inorganic insulating material is formed on the first and second gate electrodes 405 a and 405 b, and first and second active layers 415 a and 415 b each including an intrinsic polycrystalline silicon are formed on the gate insulating layer 409. The first and second active layers 415 a and 415 b are formed over the first and second gate electrodes 405 a and 405 b, respectively.

In addition, an interlayer insulating layer 422 including first to fourth active contact holes 423 a, 423 b, 423 c and 423 d is formed on the first and second active layers 415 a and 415 b. The first and second active contact holes 423 a and 423 b expos side portions of the first active layer 415 a, respectively, and the third and fourth active contact holes 423 c and 423 d expose side portions of the second active layer 415 b, respectively. First and second ohmic contact layers 427 a and 427 b of impurity-doped amorphous silicon are formed on the interlayer insulating layer 422. The first ohmic contact layer 427 a is connected to the side portions of the first active layer 415 a through the first and second active contact holes 423 a and 423 b, and the second ohmic contact layer 427 b is connected to the side portions of the second active layer 415 b through the third and fourth active contact holes 423 c and 423 d. A first source electrode 433 a and a first drain electrode 436 a spaced apart from each other are formed on the first ohmic contact layer 427 a, and a second source electrode 433 b and a second drain electrode 436 b spaced apart from each other are formed on the second ohmic contact layer 427 b. Although not shown in FIG. 8, a data line connected to the first source electrode 433 a is formed on the interlayer insulating layer 422.

The first gate electrode 405 a, the gate insulating layer 409, the first active layer 415 a, the first ohmic contact layer 427 a, the first source electrode 433 a and the first drain electrode 436 a constitute a switching thin film transistor (TFT) STr, and the second gate electrode 405 b, the gate insulating layer 409, the second active layer 415 b, the second ohmic contact layer 427 b, the second source electrode 433 b and the second drain electrode 436 b constitute a driving TFT DTr.

Further, a first passivation layer 440 of an inorganic insulating material is formed on the first source electrode 433 a, the first drain electrode 436 a, the second source electrode 433 b, the second drain electrode 436 b and the data line. The first passivation layer 440, the interlayer insulating layer 422 and the gate insulating layer 409 include first and second gate contact holes 442 a and 442 b exposing the first and second gate electrodes 405 a and 405 b, respectively. In addition, the first passivation layer 440 includes source contact hole 444 exposing the second source electrode 433 b.

A gate line 445, a gate auxiliary pattern 446 and a power electrode 434 are formed on the first passivation layer 440. The gate line 445 crosses the data line to define a pixel region P and is connected to the first gate electrode 405 a through the first gate contact hole 442 a. The gate auxiliary pattern 446 is connected to the second gate electrode 405 b through the second gate contact hole 442 b. In addition, the power electrode 434 is connected to the second source electrode 433 b through the source contact hole 444 and is connected to a power line (not shown). The power electrode 434 and the second gate electrode 405 b overlap each other with the gate insulating layer 409, the interlayer insulating layer 422 and the first passivation layer 440 interposed therebetween to constitute a first storage capacitor StgC1. Although not shown in FIG. 8, the power line parallel to the gate line 445 is formed on the first passivation layer 440 and is connected to the power electrode 434.

Moreover, a second passivation layer 450 of an inorganic insulating material is formed on the gate line 445, the gate auxiliary pattern 446 and the power electrode 434. The second passivation layer 450 includes an auxiliary contact hole 453 exposing the gate auxiliary pattern 446. In addition, the second passivation layer 450 and the first passivation layer 440 include first and second drain contact holes 452 a and 452 b exposing the first and second drain electrodes 436 a and 436 b, respectively.

A pixel electrode 470 and a gate connecting electrode 472 each including a transparent conductive material are formed on the second passivation layer 450. The pixel electrode 470 is connected to the second drain electrode 436 b through the second drain contact hole 452 b. The gate connecting electrode 472 is connected to the first drain electrode 436 a through the first drain contact hole 452 a and to the gate auxiliary pattern 446 through the auxiliary contact hole 453. In addition, the gate connecting electrode 472 overlaps the power electrode 434 with the second passivation layer 450 interposed therebetween to constitute a second storage capacitor StgC2.

Although not shown in FIG. 8, a bank of one of inorganic and organic insulating materials is formed at a boundary portion of the pixel region P. The bank may be formed on the second passivation layer 450 and the gate connecting electrode 472. In addition, an organic luminescent layer is formed on the pixel electrode 470 and a reference electrode is formed on the organic luminescent layer. The organic luminescent layer emits one of red, green and blue colored lights and the organic luminescent layers emitting different colored lights may be alternately disposed in the pixel regions P. The reference electrode may be formed over an entire surface of the substrate 401. The pixel electrode 470, the organic luminescent layer and the reference electrode in the pixel region P constitute an organic electroluminescent (EL) diode.

In the organic ELD device according to the fourth embodiment of the present invention, since the first and second active layers 415 a and 415 b include intrinsic polycrystalline silicon, a mobility of the switching TFT STr and the driving TFT DTr is improved. In addition, since the first and second gate electrodes 405 a and 405 b include impurity-doped polycrystalline silicon, the crystallization step for the first and second active layers 415 a and 415 b is performed without deformation of the first and second gate electrodes 405 a and 405 b and a polycrystalline silicon TFT having a bottom gate structure is obtained. Further, the interlayer insulating layer 422 on a central portion of the first and second active layers 415 a and 415 b functions as an etch stopper for a channel region. Accordingly, the first and second active layers 415 a and 415 b have a uniform thickness and deterioration of the switching TFT STr and the driving TFT DTr due to the thickness difference is prevented. Moreover, since the first and second ohmic contact layers 427 a and 427 b are formed of impurity-doped amorphous silicon, a doping step using an ion implantation apparatus is omitted and initial investment for an additional apparatus is reduced.

Further, the power electrode 434 connected to the second source electrode 433 b overlaps the second gate electrode 405 b with the gate insulating layer 409 and the interlayer insulating layer 422 interposed therebetween to constitute the first storage capacitor StgC1, and the gate connecting electrode 472 connected to the second gate electrode 405 b through the gate auxiliary pattern 446 overlaps the power electrode 434 extending from the second source electrode 433 b with the second passivation layer 450 interposed therebetween to constitute the second storage capacitor StgC2. Since the first and second storage capacitors StgC1 and StgC2 are connected to the second source electrode 433 b of the driving TFT DTr in parallel, the total storage capacitance of the organic ELD device increases.

FIG. 9 is a cross-sectional view showing an array substrate for an organic electroluminescent display device according to a fifth embodiment of the present invention.

In FIG. 9, a buffer layer 502 of an inorganic insulating material is formed on an entire surface of a substrate 501, and first and second gate electrodes 505 a and 505 b each including an impurity-doped polycrystalline silicon are formed on the buffer layer 502. A gate insulating layer 509 of an inorganic insulating material is formed on the first and second gate electrodes 505 a and 505 b, and first and second active layers 515 a and 515 b each including an intrinsic polycrystalline silicon are formed on the gate insulating layer 509. The first and second active layers 515 a and 515 b are formed over the first and second gate electrodes 505 a and 505 b, respectively.

In addition, an interlayer insulating layer 522 including first to fourth active contact holes 523 a, 523 b, 523 c and 523 d is formed on the first and second active layers 515 a and 515 b. The first and second active contact holes 523 a and 523 b expos side portions of the first active layer 515 a, respectively, and the third and fourth active contact holes 523 c and 523 d expose side portions of the second active layer 515 b, respectively. First and second ohmic contact layers 527 a and 527 b of impurity-doped amorphous silicon are formed on the interlayer insulating layer 522. The first ohmic contact layer 527 a is connected to the side portions of the first active layer 515 a through the first and second active contact holes 523 a and 523 b, and the second ohmic contact layer 527 b is connected to the side portions of the second active layer 515 b through the third and fourth active contact holes 523 c and 523 d. A first source electrode 533 a and a first drain electrode 536 a spaced apart from each other are formed on the first ohmic contact layer 527 a, and a second source electrode 533 b and a second drain electrode 536 b spaced apart from each other are formed on the second ohmic contact layer 527 b. In addition, a power electrode 534 is formed on the second ohmic contact layer 527 b. The second source electrode 533 b extends to the power electrode 534 and the power electrode 534 is connected to a power line (not shown). Although not shown in FIG. 9, a data line connected to the first source electrode 533 a is formed on the interlayer insulating layer 522.

The power electrode 534 and the second gate electrode 505 b overlap each other with the gate insulating layer 509 and the interlayer insulating layer 522 interposed therebetween to constitute a first storage capacitor StgC1. In addition, the first gate electrode 505 a, the gate insulating layer 509, the first active layer 515 a, the first ohmic contact layer 527 a, the first source electrode 533 a and the first drain electrode 536 a constitute a switching thin film transistor (TFT) STr, and the second gate electrode 505 b, the gate insulating layer 509, the second active layer 515 b, the second ohmic contact layer 527 b, the second source electrode 533 b and the second drain electrode 536 b constitute a driving TFT DTr.

Further, a first passivation layer 540 of an inorganic insulating material is formed on the first source electrode 533 a, the first drain electrode 536 a, the second source electrode 533 b, the second drain electrode 536 b and the data line. The first passivation layer 540, the interlayer insulating layer 522 and the gate insulating layer 509 include first and second gate contact holes 542 a and 542 b exposing the first and second gate electrodes 505 a and 505 b, respectively. In addition, the first passivation layer 540 includes a first drain contact hole 552 a exposing the first drain electrode 536 a.

A gate line 545 and a gate auxiliary pattern 546 are formed on the first passivation layer 540. The gate line 545 crosses the data line to define a pixel region P and is connected to the first gate electrode 505 a through the first gate contact hole 542 a. The gate auxiliary pattern 546 is connected to the first drain electrode 536 a through the first drain contact hole 552 b and is connected to the second gate electrode 505 b through the second gate contact hole 542 b. In addition, the gate auxiliary pattern 546 connected to the second gate electrode 505 b overlaps the power electrode 534 extending from the second source electrode 533 b with the first passivation layer 540 interposed therebetween to constitute a second storage capacitor StgC2. Although not shown in FIG. 9, the power line parallel to the gate line 545 is formed on the first passivation layer 540 and is connected to the power electrode 534 through the power contact hole.

Moreover, a second passivation layer 550 of an inorganic insulating material is formed on the gate line 545 and the gate auxiliary pattern 546. The second passivation layer 550 and the first passivation layer 540 include a second drain contact hole 552 b exposing the second drain electrode 536 b. A pixel electrode 570 including a transparent conductive material is formed on the second passivation layer 550. The pixel electrode 570 is connected to the second drain electrode 536 b through the second drain contact hole 552 b.

Although not shown in FIG. 9, a bank of one of inorganic and organic insulating materials is formed at a boundary portion of the pixel region P. The bank may be formed on the second passivation layer 550 and the gate connecting electrode 572. In addition, an organic luminescent layer is formed on the pixel electrode 570 and a reference electrode is formed on the organic luminescent layer. The organic luminescent layer emits one of red, green and blue colored lights and the organic luminescent layers emitting different colored lights may be alternately disposed in the pixel regions P. The reference electrode may be formed over an entire surface of the substrate 501. The pixel electrode 570, the organic luminescent layer and the reference electrode in the pixel region P constitute an organic electroluminescent (EL) diode.

In the organic ELD device according to the fifth embodiment of the present invention, since the first and second active layers 515 a and 515 b include intrinsic polycrystalline silicon, a mobility of the switching TFT STr and the driving TFT DTr is improved. In addition, since the first and second gate electrodes 505 a and 505 b include impurity-doped polycrystalline silicon, the crystallization step for the first and second active layers 515 a and 515 b is performed without deformation of the first and second gate electrodes 505 a and 505 b and a polycrystalline silicon TFT having a bottom gate structure is obtained. Further, the interlayer insulating layer 522 on a central portion of the first and second active layers 515 a and 515 b functions as an etch stopper for a channel region. Accordingly, the first and second active layers 515 a and 515 b have a uniform thickness and deterioration of the switching TFT STr and the driving TFT DTr due to the thickness difference is prevented. Moreover, since the first and second ohmic contact layers 527 a and 527 b are formed of impurity-doped amorphous silicon, a doping step using an ion implantation apparatus is omitted and initial investment for an additional apparatus is reduced.

Further, the power electrode 534 extending from the second source electrode 533 b overlaps the second gate electrode 505 b with the gate insulating layer 509 and the interlayer insulating layer 522 interposed therebetween to constitute the first storage capacitor StgC1, and the gate auxiliary pattern 546 connected to the second gate electrode 505 b overlaps the power electrode 534 extending from the second source electrode 533 b with the first passivation layer 540 interposed therebetween to constitute the second storage capacitor StgC2. Since the first and second storage capacitors StgC1 and StgC2 are connected to the second source electrode 533 b of the driving TFT DTr in parallel, the total storage capacitance of the organic ELD device increases.

FIG. 10 is a cross-sectional view showing an array substrate for an organic electroluminescent display device according to a sixth embodiment of the present invention.

In FIG. 10, a buffer layer 602 of an inorganic insulating material is formed on an entire surface of a substrate 601, and first and second gate electrodes 605 a and 605 b each including an impurity-doped polycrystalline silicon are formed on the buffer layer 602. A gate insulating layer 609 of an inorganic insulating material is formed on the first and second gate electrodes 605 a and 605 b, and first and second active layers 615 a and 615 b each including an intrinsic polycrystalline silicon are formed on the gate insulating layer 609. The first and second active layers 615 a and 615 b are formed over the first and second gate electrodes 605 a and 605 b, respectively.

In addition, an interlayer insulating layer 622 including first to fourth active contact holes 623 a, 623 b, 623 c and 623 d is formed on the first and second active layers 615 a and 615 b. The first and second active contact holes 623 a and 623 b expos side portions of the first active layer 615 a, respectively, and the third and fourth active contact holes 623 c and 623 d expose side portions of the second active layer 615 b, respectively. First and second ohmic contact layers 627 a and 627 b of impurity-doped amorphous silicon are formed on the interlayer insulating layer 622. The first ohmic contact layer 627 a is connected to the side portions of the first active layer 615 a through the first and second active contact holes 623 a and 623 b, and the second ohmic contact layer 627 b is connected to the side portions of the second active layer 615 b through the third and fourth active contact holes 623 c and 623 d. A first source electrode 633 a and a first drain electrode 636 a spaced apart from each other are formed on the first ohmic contact layer 627 a, and a second source electrode 633 b and a second drain electrode 636 b spaced apart from each other are formed on the second ohmic contact layer 627 b. In addition, a power electrode 634 is formed on the second ohmic contact layer 627 b. The second source electrode 633 b extends to the power electrode 634 and the power electrode 634 is connected to a power line (not shown). Although not shown in FIG. 10, a data line connected to the first source electrode 633 a is formed on the interlayer insulating layer 622.

The power electrode 634 and the second gate electrode 605 b overlap each other with the gate insulating layer 609 and the interlayer insulating layer 622 interposed therebetween to constitute a first storage capacitor StgC1. In addition, the first gate electrode 605 a, the gate insulating layer 609, the first active layer 615 a, the first ohmic contact layer 627 a, the first source electrode 633 a and the first drain electrode 636 a constitute a switching thin film transistor (TFT) STr, and the second gate electrode 605 b, the gate insulating layer 609, the second active layer 615 b, the second ohmic contact layer 627 b, the second source electrode 633 b and the second drain electrode 636 b constitute a driving TFT DTr.

Further, a first passivation layer 640 of an inorganic insulating material is formed on the first source electrode 633 a, the first drain electrode 636 a, the second source electrode 633 b, the second drain electrode 636 b and the data line. The first passivation layer 640, the interlayer insulating layer 622 and the gate insulating layer 609 include first and second gate contact holes 642 a and 642 b exposing the first and second gate electrodes 605 a and 605 b, respectively. In addition, the first passivation layer 640 includes a first drain contact hole 652 a exposing the first drain electrode 636 a.

A gate line 645 and a gate auxiliary pattern 646 are formed on the first passivation layer 640. The gate line 645 crosses the data line to define a pixel region P and is connected to the first gate electrode 605 a through the first gate contact hole 642 a. The gate auxiliary pattern 646 is connected to the first drain electrode 636 a through the first drain contact hole 652 b and is connected to the second gate electrode 605 b through the second gate contact hole 642 b. In addition, the gate auxiliary pattern 646 connected to the second gate electrode 605 b overlaps the power electrode 634 extending from the second source electrode 633 b with the first passivation layer 640 interposed therebetween to constitute a second storage capacitor StgC2. Although not shown in FIG. 10, the power line parallel to the gate line 645 is formed on the first passivation layer 640 and is connected to the power electrode 634 through the power contact hole.

Moreover, a second passivation layer 650 of an inorganic insulating material is formed on the gate line 645 and the gate auxiliary pattern 646. The second passivation layer 650 and the first passivation layer 640 include a second drain contact hole 652 b exposing the second drain electrode 636 b and a source contact hole 662 exposing the second source electrode 633 b. A pixel electrode 670 and a capacitor auxiliary pattern 680 each including a transparent conductive material are formed on the second passivation layer 650. The pixel electrode 670 is connected to the second drain electrode 636 b through the second drain contact hole 652 b, and the capacitor auxiliary pattern 680 is connected to the second source electrode 633 b through the source contact hole 662. In addition, the capacitor auxiliary pattern 680 connected to the second source electrode 633 b overlaps the gate auxiliary pattern 646 connected to the second gate electrode 605 b with the second passivation layer 650 interposed therebetween to constitute a third storage capacitor StgC3.

Although not shown in FIG. 10, a bank of one of inorganic and organic insulating materials is formed at a boundary portion of the pixel region P. The bank may be formed on the second passivation layer 650 and the gate connecting electrode 672. In addition, an organic luminescent layer is formed on the pixel electrode 670 and a reference electrode is formed on the organic luminescent layer. The organic luminescent layer emits one of red, green and blue colored lights and the organic luminescent layers emitting different colored lights may be alternately disposed in the pixel regions P. The reference electrode may be formed over an entire surface of the substrate 601. The pixel electrode 670, the organic luminescent layer and the reference electrode in the pixel region P constitute an organic electroluminescent (EL) diode.

In the organic ELD device according to the sixth embodiment of the present invention, since the first and second active layers 615 a and 615 b include intrinsic polycrystalline silicon, a mobility of the switching TFT STr and the driving TFT DTr is improved. In addition, since the first and second gate electrodes 605 a and 605 b include impurity-doped polycrystalline silicon, the crystallization step for the first and second active layers 615 a and 615 b is performed without deformation of the first and second gate electrodes 605 a and 605 b and a polycrystalline silicon TFT having a bottom gate structure is obtained. Further, the interlayer insulating layer 622 on a central portion of the first and second active layers 615 a and 615 b functions as an etch stopper for a channel region. Accordingly, the first and second active layers 615 a and 615 b have a uniform thickness and deterioration of the switching TFT STr and the driving TFT DTr due to the thickness difference is prevented. Moreover, since the first and second ohmic contact layers 627 a and 627 b are formed of impurity-doped amorphous silicon, a doping step using an ion implantation apparatus is omitted and initial investment for an additional apparatus is reduced.

Further, the power electrode 634 extending from the second source electrode 633 b overlaps the second gate electrode 605 b with the gate insulating layer 609 and the interlayer insulating layer 622 interposed therebetween to constitute the first storage capacitor StgC1, the gate auxiliary pattern 646 connected to the second gate electrode 605 b overlaps the power electrode 634 extending from the second source electrode 633 b with the first passivation layer 640 interposed therebetween to constitute the second storage capacitor StgC2, and the capacitor auxiliary pattern 680 connected to the second source electrode 633 b overlaps the gate auxiliary pattern 646 connected to the second gate electrode 605 b with the second passivation layer 650 interposed therebetween to constitute the third storage capacitor StgC3. Since the first, second and third storage capacitors StgC1, StgC2 and StgC3 are connected to the second source electrode 633 b of the driving TFT DTr in parallel, the total storage capacitance of the organic ELD device increases.

Although an array substrate for an organic electroluminescent display device is illustrated in first to sixth embodiments of the present invention, the structure and the fabricating method of the TFT of the first to sixth embodiments can be applied to an array substrate for another display device such as a liquid crystal display device. For example, the pixel electrode may be connected to the switching TFT without the driving TFT in the array substrate for an LCD device.

It will be apparent to those skilled in the art that various modifications and variations can be made in the method of fabricating a display device using a plastic substrate of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents. 

1. An array substrate for a display device, comprising: a substrate; first and second gate electrodes of impurity-doped polycrystalline silicon on the substrate; a gate insulating layer on the first and second gate electrodes; first and second active layers of intrinsic polycrystalline silicon on the gate insulating layer, the first and second active layers corresponding to the first and second active layers, respectively; an interlayer insulating layer on the first and second active layers and including first to fourth active contact holes, the first and second active contact holes exposing side portions of the first active layer, the third and fourth active contact holes exposing side portions of the second active layer; first and second ohmic contact layers of impurity-doped amorphous silicon on the interlayer insulating layer, the first ohmic contact layer contacting the first active layer through the first and second active contact holes, the second ohmic contact layer contacting the second active layer through the third and fourth active contact hole; first source and drain electrodes on the first ohmic contact layer and second source and drain electrodes on the second ohmic contact layer; a data line on the interlayer insulating layer, the data line connected to the first source electrode; a first passivation layer on the first source and drain electrodes, the second source and drain electrodes and the data line; a gate line on the first passivation layer, the gate line connected to the first gate electrode and crossing the data line to define a pixel region; a second passivation layer on the gate line; and a pixel electrode on the second passivation layer, the pixel electrode connected to the second drain electrode.
 2. The substrate according to claim 1, further comprising: a power electrode connected to the second source electrode; and a power line and a gate auxiliary pattern on the first passivation layer, the power line connected to the power electrode, the gate auxiliary pattern connected to the second gate electrode.
 3. The substrate according to claim 2, wherein the gate insulating layer, the interlayer insulating layer and the first passivation layer include first and second gate contact holes exposing the first and second gate electrodes, respectively, wherein the gate line is connected to the first gate electrode through the first gate contact hole and the gate auxiliary pattern is connected to the second gate electrode through the second gate contact hole.
 4. The substrate according to claim 3, further comprising a gate connecting electrode on the second passivation layer, wherein the first and second passivation layers include first and second drain contact holes exposing the first and second drain electrodes, respectively, and the second passivation layer includes an auxiliary contact hole exposing the gate auxiliary pattern, and wherein the gate connecting electrode is connected to the first drain electrode through the first drain contact hole and to the gate auxiliary pattern through the auxiliary contact hole, and the pixel electrode is connected to the second drain electrode through the second drain contact hole.
 5. The substrate according to claim 4, wherein the power electrode is formed on the interlayer insulating layer and extends from the second source electrode, and the power electrode overlaps the second gate electrode to constitute a first storage capacitor with the gate insulating layer and the interlayer insulating layer interposed therebetween.
 6. The substrate according to claim 5, wherein the gate auxiliary pattern overlaps the power electrode to constitute a second storage capacitor with the first passivation layer interposed therebetween.
 7. The substrate according to claim 5, wherein the gate connecting electrode overlaps the power electrode to constitute a second storage capacitor with the first and second passivation layers interposed therebetween.
 8. The substrate according to claim 4, wherein the power electrode is formed on the first passivation layer and is connected to the second source electrode through a source contact hole in the first passivation layer, wherein the power electrode overlaps the second gate electrode to constitute a first storage capacitor with the gate insulating layer, the interlayer insulating layer and the first passivation layer interposed therebetween, and wherein the gate connecting electrode overlaps the power electrode to constitute a second storage capacitor with the second passivation layer interposed therebetween.
 9. The substrate according to claim 3, wherein the first passivation layer includes a first drain contact hole exposing the first drain electrode, and the first and second passivation layers include a second drain contact hole exposing the second drain electrode, and wherein the gate auxiliary pattern is connected to the first drain electrode through the first drain contact hole, and the pixel electrode is connected to the second drain electrode through the second drain electrode.
 10. The substrate according to claim 9, wherein the power electrode is formed on the interlayer insulating layer and extends from the second source electrode, and the power electrode overlaps the second gate electrode to constitute a first storage capacitor with the gate insulating layer and the interlayer insulating layer interposed therebetween, and wherein the gate auxiliary pattern overlaps the power electrode to constitute a second storage capacitor with the first passivation layer interposed therebetween.
 11. The substrate according to claim 10, further comprising a capacitor auxiliary pattern on the second passivation layer and connected to the second source electrode through a source contact hole in the first and second passivation layers, wherein the capacitor auxiliary pattern overlaps the gate auxiliary pattern to constitute a third storage capacitor with the second passivation layer interposed therebetween.
 12. The substrate according to claim 1, further comprising a buffer layer between each of the first and second gate electrodes and the substrate, wherein the buffer layer includes an inorganic insulating material.
 13. The substrate according to claim 1, further comprising a barrier layer under the first and second ohmic contact layers, wherein the barrier layer includes intrinsic amorphous silicon and has the same shape as the first and second ohmic contact layers.
 14. A method of fabricating an array substrate for a display device, comprising: sequentially forming a first impurity-doped amorphous silicon layer, a first inorganic insulating layer and an intrinsic amorphous silicon layer on a substrate; crystallizing the first impurity-doped amorphous silicon layer and the intrinsic amorphous silicon layer to form an impurity-doped polycrystalline silicon layer and an intrinsic polycrystalline silicon layer; patterning the impurity-doped polycrystalline silicon layer, the first organic insulating layer and the intrinsic polycrystalline silicon layer to form a first gate electrode, a gate insulating layer and a first active layer of polycrystalline silicon sequentially on the substrate and a second gate electrode, the gate insulating layer and a second active layer of polycrystalline silicon sequentially on the substrate; forming an interlayer insulating layer on the first and second active layers, the interlayer insulating layer including first to fourth active contact holes, the first and second active contact holes exposing side portions of the first active layer, the third and fourth active contact holes exposing side portions of the second active layer; sequentially forming a second impurity-doped amorphous silicon layer and a first metal layer on the interlayer insulating layer; pattering the second impurity-doped amorphous silicon layer and the first metal layer to form first and second ohmic contact layers, first source and drain electrodes on the first ohmic contact layer, second source and drain electrodes on the second ohmic contact layer and a data line on the interlayer insulating layer, the first ohmic contact layer contacting the first active layer through the first and second active contact holes, the second ohmic contact layer contacting the second active layer through the third and fourth active contact hole; forming a first passivation layer on the first source and drain electrodes, the second source and drain electrodes and the data line; forming a gate line on the first passivation layer, the gate line connected to the first gate electrode and crossing the data line to define a pixel region; forming a second passivation layer on the gate line; and forming a pixel electrode on the second passivation layer, the pixel electrode connected to the second drain electrode. 